The electronic absorption spectra of 2-hydroxy-l,4-naphthoquinone (HNQ) in one- and
two-component solvents are discussed. The visible absorption of this reagent has been shown to arise
from a charge-transfer transition of the zwitterionic tautomer stabilized by a hydrogen bond donating
solvent. The formation constant of the 1 : 1 hydrogen bonded complex with methanol, ethanol, and
n-propanot has been determined at 25 °C from the spectral behaviour in mixed solvents. The
tautomerization equilibrium of HNQ in aqueous ethanol is demonstrated and characterized. The
dependence of the absorbance maximum of HNQ on the hydrogen bond donor ability of the solvent in
the visible spectrum, as measured by the Taft-Kamlet ~ parameter, has been established. Experimentally derived evidence is supplied to justify the recommendation of HNQ as a novel probe for hydrogen
bond donor ability in pure solvents and in mixed aqueous solvents.